134 Transactions of the Society. 



XL — Tlie Aperture Question. By J. Mayall, jun., F.E.M.S. 



i^Reud Sth January, 1879.) 



The question of the existence of apertures, by means of the immer- 

 sion system, greater than correspond to the maximum possible for 

 dry lenses, has received such powerful support in the affirmative 

 from Zeiss's new oil lenses, that it is almost superfluous to call atten- 

 tion to the position of the discussion. But as the chief exponent of 

 the adverse view still maintains that it is an " undecided question," 

 I will briefly state the most obvious points that occur to me. 



It had been asserted by Mr. Wenham that 82" in the body of 

 the front lens is the limit beyond which no object-glass can collect 

 image-forming rays. I quote a passage from his writings to show 

 that he has clearly pledged himself that this limit obtains equally in 

 dry and immersion lenses on balsamed objects : — 



" . . . . the immersion lens .... had no property for col- 

 lecting from a balsam-mounted object a greater number of rays, but 

 that the limit is the same as in the dry lens." 



Many passages might be cited conveying the same view. 



This is equivalent to asserting the existence of a natural limit, 

 depending on twice the critical angle (from glass to air i, and, con- 

 sequently, the impossibility of any objective collecting to a focus 

 a pencil of rays from a radiant in balsam of greater aperture than 

 that which in this medium corresponds to 180° in air. It was to 

 this assertion as regards the limit in relation to immersion lenses 

 that exception was taken. 



On this question Professor Stokes was urged by me to give a 

 demonstration, and I think it must be admitted that the assertion is 

 thereby refuted as a question of theory. Mr. Wenham admits the 

 validity of the reasoning, but insists that in practical constructions 

 the limit of 82° obtains. 



Mr. Wenham's views had been brought to definite issue by his 

 published report of his measurement of the aperture of Tolles's i 

 immersion lens (owned by Mr. Crisp). The constructor had al- 

 leged the lens to be made on a formula by which an aperture was 

 obtained, measured in the body of the front lens (or in a suitably 

 adjusted semi-cylinder — for it is demonstrable that the results are 

 equivalent), 16° beyond the maximum possible for dry lenses, — that 

 is to say, Mr. Tolles claimed for it an aperture in glass of 98°. 



Mr. Wenham reported the aperture to be 68° in glass. 



The point of interest to me was to prove whether the aperture 

 exceeded 82°. 



Passing over some discussions that took place in correspondence, 

 •which were not communications to the Society, I may state that I 

 felt under an obligation to place before the Society the evidence I 



